A variety of optical systems and devices use laser light. Many types of lasers are well-known for emitting light with a stable output frequency ν while exhibiting a narrow linewidth Δν and a long coherence length Lc. In fact, narrow linewidth implies large coherence length Lc since the relationship between coherence length Lc and linewidth Δν is:
            L      c        =          c              Δ        ⁢                                  ⁢        v              ,where c is the speed of light.
A number of applications require light that has a stable output frequency ν but a broad linewidth Δν. More specifically, they require light whose optical output frequency is constant or has a known stochastic property over time yet simultaneously spans a broad frequency range (i.e., has a large bandwidth).
To comply with the first requirement of stable emission frequency ν the art teaches laser cavities that have a high finesse and carefully controlled geometry. Unfortunately, the byproduct of cavities with high finesse is a narrow emission linewidth. Of course, the linewidth depends also on the laser type and cavity configuration, but it is still often less than 1-10 MHz and thus too narrow for many applications.
Numerous optical systems using light with a large coherence length Lc can suffer from unwanted interference fringes. That is because such systems have a number of optical surfaces that support reflection and can thus produce undesired interference patterns. This undesired interference, also referred to as “ghost fringes”, creates spurious signals that can wash out the desired fringes or degrade the accuracy of measurement.
Interferometry is among the many applications in which a controlled coherence length Lc is desirable. An interferometer requires coherence only over a limited distance such that interference fringes are created just between the optical surfaces of interest. These surfaces are often close to one another. Thus, it is desirable for the optical path difference (OPD) between the surfaces of interest to be roughly the same as the coherence length Lc.
To produce light exhibiting stable frequency and broad linewidth the prior art teaches the use of inherently broad sources, such as superluminescent diodes (SLDs), light emitting diodes (LEDs), Fabry-Perot diodes (FPDs) or emission sources such as mercury lamps. These sources can have very short coherence lengths, e.g., on the order of microns, but suffer from one or more of the following disadvantages: Their coherence length Lc is not readily adjustable, since it is an inherent characteristic of the light source; their output power is low (such as the SLD); or they exhibit low spatial coherence (in particular, LEDs). Finally, they can exhibit a high sideband content (e.g., FPDs), which is inhomogeneous and can result in ghost fringes or washed-out fringes.
The prior art also teaches frequency shifting with the aid of devices such as discrete electro-optical modulators (EOMs) driven at a particular frequency to add sidebands beyond the linewidth. There are numerous teachings on appropriate resonant electronic circuitry to drive EOMs to create sidebands such as U.S. Pat. No. 5,189,547 to Day et al. Sidebands effectively shorten the average coherence length Lc and can thus be used to broaden the linewidth as taught, e.g., by A. Dunbar et al., “Extended ATHENA Alignment Performance and Application for the 100 nm Technology Node”, ASML (Netherlands), Proceedings of SPIE, Vol. 4344,Metrology, Inspection, and Process Control for Microlithography XV, paper #4344-85 (2001). For related teachings on the use of EOMs or electroabsorption modulators for creating sidebands, harmonics and combs of spaced frequencies as well as frequency shifting the reader is referred to U.S. Pat. Nos. 5,917,636; 5,621,744; 5,434,693 and U.S. Application No. 2002/0196509.
Unfortunately, the presence of discrete sidebands will cause strong interference at optical path differences (OPDs) described by:
  OPD  =      c          v      m      where νm is the modulation frequency. Thus, if any optical surfaces of the optical system capable of producing interference happen to have this spacing, then corresponding strong interference fringes will appear. This problem can be solved, albeit in a cumbersome manner, by carefully designing the spacings of elements in the optical system and/or adjusting the modulation frequency.
Yet another approach taught in the art is the use of in-cavity linewidth-broadening elements such as acousto-optic modulators (AOMs). In this approach the linewidth is varied continuously by adjusting the RF power delivered to the AOM. For teachings on how AOMs and/or acousto-optic tunable filters (AOTFs) can be used in various in-cavity configurations including external cavity diode lasers (ECDLs) the reader is referred to F. V. Kowalski et al., “Broadband Continuous-Wave Laser”, Optics Letters, Vol. 13,pp. 622-4 (1988); P. I. Richter and T. W. Hansch, “Diode Lasers in External Cavities with Frequency-Shifted Feedback”, Optics Communications, Vol. 85,pp. 414-418 (1991); A. P. Willis et al., “External Cavity Laser Diodes with Frequency-Shifted Feedback”, Optics Communications, Vol. 116, pp. 87-93 (1995); and M. J. Lim et al., “Improved Design of a Frequency-Shifted Feedback Diode Laser for Optical Pumping at High Magnetic Field”, Optical Communications, Vol. 147,pp. 99-102 (1998).
The primary disadvantages of this technique are that it requires a custom-designed ECDL and that the broadening is limited by the available frequencies of AOMs and their power-handling capability. Furthermore, the introduction of the AOM into the cavity reduces the power output of the laser.
Still another prior art approach to broaden the linewidth teaches radio-frequency (RF) modulation of a laser diode current to introduce sidebands. For more specific information on this approach the reader is referred to Paul Feng and Thad Walker, “Inexpensive Diode Laser Microwave Modulation for Atom Trapping”, American Journal of Physics, Vol. 63 (No. 10), pp. 905-8 (1995) and R. Kowalski et al., “A Frequency-Modulated Injection-Locked Diode Laser for Two-Frequency Generation”, Review of Scientific Instruments, Vol. 72, No. 6, pp. 2532-4 (2001).
Besides the above-discussed disadvantages caused by discrete sidebands, this approach is limited in that it only works with laser diodes and furthermore the modulation frequency is limited by the bandwidth of the laser diode itself and by the diode packaging. Furthermore, modulation of the laser diode current often introduces undesirable residual amplitude modulation (RAM) of the laser output power.
Still another approach to broadening a beam of laser light is addressed by Storz in U.S. Application 2002/0043618. Storz teaches to use an electro-optic modulator as a phase modulation means for varying the phase angle of light in a confocal microscope. Storz indicates that the noise signal can be periodic or stochastic. Unfortunately, this manner of applying noise to an electro-optic modulator, although capable of broadening the emission linewidth of a laser, is not capable of homogeneously broadening it in a controlled manner, which is what the art requires.
Therefore, there still exists an unsolved problem of broadening the emission linewidth of lasers in a homogeneous and simple manner.